The present invention relates to an improved shaping aperture for shaping a charged particle beam, which is used in a charged particle forming system included in an electron beam lithography system and the like, into a desired form.
FIG. 1 of the accompanying drawings shows an example of a column (that is, an electron-optical column) included in an electron beam lithography system in which an electron beam is shaped into a desired form and the electron beam thus shaped is employed to write various patterns on a semiconductor substrate (that is, a semiconductor wafer), or to expose various surface portions of a glass mask plate. In FIG. 1, reference numeral 1 designates an electron gun, 2 a condenser lens, 3 a first shaping aperture, 4 an electrostatic deflector, 5 a projection lens, 6 a second shaping aperture, 7 an electrostatic deflector for scanning operation, and 8 a target plane or a sample to be exposed.
Usually in an electron beam lithography system and the like, the shaping apertures 3 and 6 are mounted and fixed in the column, as shown in FIG. 1. FIGS. 2a and 2b show, in an enlarged form, one of the shaping apertures 3 and 6. Referring to FIG. 2a, an electron beam 9 (having a circular cross section) emitted from the electron gun 1 illuminates a shaping aperture 10 in such a manner that a shaping slit 11 (having the form of a square in the example shown in FIGS. 2a and 2b) provided in the shaping aperture 10 is covered with a large region 12 in which the current density is uniform. A shaped electron beam 13 which has passed through the shaping slit 11, is focussed and deflected by means of an electromagnetic lens or the like provided in the succeeding stage, and illuminates the target 8 with a desired demagnification, in order to write a pattern thereon. In many cases, the shaping aperture 10 shown in FIG. 2a is made of a material such as Mo, Pt and Cu, and is formed of a thin plate having a diameter of about 6 mm and a thickness of 10 to 20 .mu.m. The shaping slit 11 measures about 300 .mu.m.times.300 .mu.m. When a slit having a size smaller than 500 .mu.m.times.500 .mu.m is formed by machining, it is very difficult, if not impossible, to find a cutting method for forming a slit having accurate dimensions.
Accordingly, the operation for providing such a through-hole is performed mainly through photoetching techniques. As a result, as shown in FIG. 2b, there is obtained the slit 11 having corners which are considerably rounded. For example, in the case where the side a of the slit has a length of 250 .mu.m, the radius of curvature of each corner lies within a range from 10 to 30 .mu.m. Such roundness is undesirable, since the roundness is left as a part of a demagnified writing PG,4 pattern when the demagnified writing pattern is projected onto the target 8. For example, in the case where the above-mentioned side a is 250 .mu.m in length and the total demagnification is made equal to 1/50, the corners (of the slit) having a radius of curvature of 10 .mu.m will produce on the target 8 a pattern having a radius of curvature of 0.2 .mu.m. Accordingly, the roundness of slit corner cannot be neglected when it is required to write an ultra fine pattern, a minimum width of which is less than 1 .mu.m, with high accuracy.